Regulation of cell activities can involve the binding of a ligand to a membrane-bound receptor comprising an extracellular ligand binding domain and an intracellular (e.g., cytoplasmic) signaling domain. The formation of a complex between a ligand and the ligand binding domain can result in a conformational and/or chemical modification in the receptor which can result in a signal transduced within the cell. In some situations, the cytoplasmic portion of the receptor is phosphorylated (e.g., trans- and/or auto-phosphorylated), resulting in a change in its activity. These events can be coupled with secondary messengers and/or the recruitment of co-factor proteins. In some instances, the change in the cytoplasmic portion results in binding to other proteins (e.g., co-factor proteins and/or other receptors). These other proteins can be activated and then carry out various functions within a cell.
Conditional gene expression systems allow for conditional regulation of one or more target genes. Conditional gene expression systems such as drug-inducible gene expression systems allow for the activation and/or deactivation of gene expression in response to a stimulus, such as the presence of a drug. Currently available systems, however, can be limited due to imprecise control, insufficient levels of induction (e.g., activation and/or deactivation of gene expression), and lack of specificity.